Congestion-based network traffic policing based on congestion exposure is a promising network resource control paradigm that accounts for user traffic in the event of network congestion. It has been argued, for example in B. Briscoe, “Flow Rate Fairness Dismantling a Religion”, ACM Computer Communications Review, 37(2), 63-74 (April 2007), that flow rate fairness, which has been used in the past, is not a reasonable mechanism for resource allocation and accountability of network resources. Instead, it is suggested that a cost-based mechanism would provide a better resource allocation paradigm, in which “cost” means the degree to which each user's transfers restrict other transfers, given the available network resources. The metric that has been suggested for measuring this cost is the volume of network congestion caused by each user. A network traffic policing mechanism based on congestion offers a net-neutral way for network operators to manage traffic on their networks.
There have been several proposals for implementing congestion-based network traffic policing. For example, Re-ECN (Relay or Re-feedback of Explicit Congestion Notification) is a proposal that has been made in the Internet Engineering Task Force (IETF) Congestion Exposure (CONEX) Working Group, being described, for example, in B. Briscoe, A. Jacquet, C. Di Cairano-Gilfedder, A. Salvatori, A. Soppera, and M. Koyabe, “Policing Congestion Response in an Internetwork using Re-feedback”, Proc. ACM SIGCOMM'05, CCR, 35(4):277-288, August 2005. As will be discussed in greater detail below, Re-ECN, or re-feedback of explicit congestion notification provides a feedback mechanism through which packets expose the congestion that they expect to cause. The key feature is the user based accountability that is not based on resource usage but on the congestion user traffic is causing to others in a network.
For instance, in a Re-ECN system as shown in FIG. 1, which is a specific implementation of a congestion exposure system, there are different functional entities: routers detect congestion and apply Explicit Congestion Notification (ECN) to packets in their queue. Receiving endpoints collect this congestion information and relay it back to the sender. The sender runs a transport protocol (for instance TCP) and can use this information for the congestion control algorithm of a transport protocol. Also, the sender is expected to re-act to the received feedback declaring its contribution to congestion for subsequent packet transmissions. This is done by marking a certain fraction of packets appropriately. An operator-provided Policer can use this information to police or to account for traffic accordingly.
For measuring conformant congestion response, congestion exposure system normally rely on an audit function placed at the network egress. For instance, in Re-ECN the audit function is implemented in form of a packet dropper entity. Therefore, packet dropper and audit function are used as synonyms hereinafter. Based on the actual observed path congestion and based on the declared congestion contribution by sender, such an audit function/packet dropper can enforce the overall congestion-based policing by dropping packets pertaining to particular non-conformant flows.
Under congestion situations, for a given response there is an upper limit on the possible bit rate offered by the network. Naturally, users want to increase their bit rate (utility) by understating their congestion contribution in the network. Such a non-conformant behavior is termed as cheating because under the given network congestion it offers a higher bit rate, i.e. network-resources, than those users which are declaring congestion honestly, i.e. in a conformant fashion. Thus, although congestion exposure systems are a promising network resource control approach the problem remains that they require joint participation from end hosts and a network operator.
Collectively, the path congestion and its response can best be estimated at the network egress. The challenge remains that the cheating users are hidden in the overall traffic aggregate. By varying the response, users can devise rich set of cheating patterns that a network must identify and penalize. Since the congestion declaration is under the full control of the sending host, an operator must monitor user response to the network congestion for fair resource accountability among the competing hosts.